A standard 12-lead electrocardiogram (ECG) is a representation of the heart's electrical activity recorded from electrodes on the body surface. Since its invention by Willem Einthoven in 1904, ECG has been a standard diagnosis tool for evaluating cardiac function. Normal ECG tracing is comprised of different waves that represent a sequence of depolarization and repolarization of the atria and ventricles. For example, a P wave represents atrial depolarization, a QRS complex represents ventricular depolarization, and a T wave represents ventricular repolarization. From these ECG waves, a plurality of intervals may be calculated that reflect cardiac conduction properties (e.g., P wave duration, PR interval, and QRS duration), repolarization properties (e.g., QT interval), intrinsic heart rate (e.g., PP or RR intervals), etc. Collectively, these ECG waves and durations contain important diagnostic information regarding a patient's underlying cardiac condition.
However, many patients experience intermittent spontaneous cardiac arrhythmias such as, for example, sinus bradycardia, non-sustained ventricular tachycardia, or paroxysmal atrial fibrillation, events which may not be recorded during their clinic visits. In order to capture these infrequent arrhythmia episodes, external ECG monitoring devices such as Holter monitors are often prescribed to continuously monitor the patient's ECG. However, Holter recording has two inherent drawbacks. First, the memory capacity is limited, and most commercially available Holter machines can only record 24-hr or 48-hr surface ECG measurements. Second, the use of skin electrodes is inconvenient and uncomfortable for the patient, and is a significant source of measurement noise due to loose contact, muscle movement, and environmental factors. Consequently, the diagnostic yield of a Holter ECG is very limited.
To overcome these shortcomings, subcutaneous ECG monitors have been introduced. By implanting a small device with sensing electrodes underneath the skin, the subcutaneous ECG monitor can record a subcutaneous ECG that resembles the surface ECG. The subcutaneous ECG monitor can be configured as a loop recorder, so that it continuously records newly acquired subcutaneous ECG signals while discarding the old recordings. When experiencing symptoms, a patient can use a handheld device that communicates with the subcutaneous device to trigger a snapshot of the recordings. Alternatively, the implantable device can be programmed to automatically trigger a snapshot of the subcutaneous ECG upon detection of an arrhythmic episode. The recorded snapshots can then be transmitted over the telephone or over a wireless network to a physician's office for clinical review. Because the loop recorder continuously refreshes its memory, it can be carried for long periods of time. Thus it is ideal for capturing ECG traces of infrequent episodes such as syncope. Recently, subcutaneous ECG recording has also become a useful means to monitor the cardiac rhythm after ablation of atrial fibrillation, to determine the ablation efficacy and to adjust therapeutic treatments.
Regardless of the specific ECG recording apparatus (e.g., ECG machines, bedside ECG monitors, Holter ECG monitors, subcutaneous ECG devices), reliable beat detection is the prerequisite for further ECG processing and clinical diagnosis. Despite decades of research, ECG beat detection has remained a technical challenge. On the one hand, many factors can cause over-sensing (false detection) of cardiac beats, such as large T waves, wide QRS complexes, muscle noise, electromagnetic interference (EMI), and the like. On the other hand, under-sensing (missed detection) of cardiac beats is also common for ECG signals that have a small signal to noise ratio. Existing methods for real-time ECG beat detection are either computationally complex, and therefore not suitable for implementation in an embedded system, or oversimplified so that they rely solely on ECG metrics such as peak amplitude, peak slope, etc., with or without an adaptive sensing threshold, and thus they result in unsatisfactory performance.
For at least the reasons given above, there is a need to provide a method and apparatus for more accurate and efficient detection of cardiac beats in response to surface ECG or subcutaneous ECG recordings.